Gasoline typically includes a mixture of hydrocarbons ranging from higher volatility butanes (C4) to lower volatility C8 to C10 hydrocarbons. When vapor pressure increases in the fuel tank due to conditions such as higher ambient temperature or displacement of vapor during filling of the tank, fuel vapor flows through openings in the fuel tank. To prevent fuel vapor loss into the atmosphere, the fuel tank is vented into a canister that contains an adsorbent material such as activated carbon granules (“evap” canister).
The fuel vapor is a mixture of the gasoline vapor (referred to in this description also by its main component, hydrocarbon vapor) and air. As the fuel vapor enters an inlet of the canister, the hydrocarbon vapor is adsorbed onto activated carbon granules and the air escapes into the atmosphere. The size of the canister and the volume of the adsorbent activated carbon are selected to accommodate the expected gasoline vapor generation. After the engine is started, the control system uses engine intake vacuum to draw air through the adsorbent to desorb the fuel. The desorbed fuel vapor is directed into an air induction system of the engine as a secondary air/fuel mixture. One exemplary evaporative control system is described in U.S. Pat. No. 6,279,548 to Reddy, which is hereby incorporated by reference.
When the gasoline tank is filled, fuel vapor accumulates in the canister. The initial loading may be at the inlet end of the canister, but over time the fuel vapor is gradually distributed along the entire bed of the adsorbent material. After the engine is started, a purge valve is opened and air is drawn through the canister. The air removes fuel vapor that is stored in the adsorbent material.
One problem encountered by such a system has been vapor breakthrough, or hydrocarbon emissions from the vented vapor adsorption canister, which is often referred to as canister bleed emissions. Such emissions may be, for example, about 20 mg hydrocarbons per day. It would be desirable to substantially decrease the bleed emissions from the vapor emission control system.